Many distributions will only provide security and stability updates to Firefox. They will, for example, let you go from 1.5.0.10 to 1.5.0.11 but not to 2.0. To make a major upgrade, you will have to install Firefox outside of your package manager. See Install Firefox on Linux.

Permissions

If your WindowsMac OSoperating system account doesn't have permission to change the Firefox installation directory, updating Firefox will not work, so Check for Updates... is disabled. To get Firefox updated, log in to your computer as a user with sufficient permissions, for example an administrative account. Check for Updates... should then be enabled.